GSM spectrum: Invisible 200kHz carriers?

Hi,

I am using the USRP board with the DBSRX daughter board.

According to my knowledge, the swiss GSM 900 frequency band consists of
channels at an interval of 200kHz. However, when I build a simple system
with an USRP source and a FFT sink and analyze the spectrum around
900MHz, I
only see peaks spaced 4MHz (about 30dB above noise level). I also tried
to
set the USRP gain to 100dB, the shape of the frequency plot didn’t
change.

Is it possible to see these 200kHz channels without further filtering?
Or do
I miss an important detail in how these 4MHz peaks come about?

Thanks in advance.

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